Light sensitive display

ABSTRACT

A light sensitive display.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a division of U.S. patent application Ser. No.10/217,798, filed Aug. 12, 2002, which claims the benefit of U.S.Provisional App. No. 60/359,263, filed Feb. 2, 2002.

BACKGROUND OF THE INVENTION

The present invention relates to touch sensitive displays.

Touch sensitive screens (“touch screens”) are devices that typicallymount over a display such as a cathode ray tube. With a touch screen, auser can select from options displayed on the display's viewing surfaceby touching the surface adjacent to the desired option, or, in somedesigns, touching the option directly. Common techniques employed inthese devices for detecting the location of a touch include mechanicalbuttons, crossed beams of infrared light, acoustic surface waves,capacitance sensing, and resistive membranes.

For example, Kasday, U.S. Pat. No. 4,484,179 discloses anoptically-based touch screen comprising a flexible clear membranesupported above a glass screen whose edges are fitted with photodiodes.When the membrane is flexed into contact with the screen by a touch,light which previously would have passed through the membrane and glassscreen is trapped between the screen surfaces by total internalreflection. This trapped light travels to the edge of the glass screenwhere it is detected by the photodiodes which produce a correspondingoutput signal. The touch position is determined by coordinating theposition of the CRT raster beam with the timing of the output signalsfrom the several photodiodes. The optically-based touch screen increasesthe expense of the display, and increases the complexity of the display.

Denlinger, U.S. Pat. No. 4,782,328 on the other hand, relies onreflection of ambient light from the actual touch source, such as afinger or pointer, into a pair of photosensors mounted at corners of thetouch screen. By measuring the intensity of the reflected light receivedby each photosensor, a computer calculates the location of the touchsource with reference to the screen. The inclusion of the photosensorsand associated computer increases the expense of the display, andincreases the complexity of the display.

May, U.S. Pat. No. 5,105,186, discloses a liquid crystal touch screenthat includes an upper glass sheet and a lower glass sheet separated byspacers. Sandwiched between the glass sheets is a thin layer of liquidcrystal material. The inner surface of each piece of glass is coatedwith a transparent, conductive layer of metal oxide. Affixed to theouter surface of the upper glass sheet is an upper polarizer whichcomprises the display's viewing surface. Affixed to the outer surface ofglass sheet is a lower polarizer. Forming the back surface of the liquidcrystal display is a transflector adjacent to the lower polarizer. Atransflector transmits some of the light striking its surface andreflects some light. Adjacent to transflector is a light detecting arrayof light dependent resistors whose resistance varies with the intensityof light detected. The resistance increases as the light intensitydecreases, such as occurs when a shadow is cast on the viewing surface.The light detecting array detect a change in the light transmittedthrough the transflector caused by a touching of viewing surface.Similar to touch sensitive structures affixed to the front of the liquidcrystal stack, the light sensitive material affixed to the rear of theliquid crystal stack similarly pose potential problems limiting contrastof the display, increasing the expense of the display, and increasingthe complexity of the display.

Touch screens that have a transparent surface which mounts between theuser and the display's viewing surface have several drawbacks. Forexample, the transparent surface, and other layers between the liquidcrystal material and the transparent surface may result in multiplereflections which decreases the display's contrast and produces glare.Moreover, adding an additional touch panel to the display increases themanufacturing expense of the display and increases the complexity of thedisplay. Also, the incorporation of the touch screen reduces the overallmanufacturing yield of the display.

Accordingly, what is desired is a touch screen that does notsignificantly decrease the contrast ratio, does not significantlyincrease the glare, does not significantly increase the expense of thedisplay, and does not significantly increase the complexity of thedisplay.

The foregoing and other objectives, features, and advantages of theinvention will be more readily understood upon consideration of thefollowing detailed description of the invention, taken in conjunctionwith the accompanying drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a cross sectional view of a traditional active matrix liquidcrystal display.

FIG. 2 is a schematic of the thin film transistor array.

FIG. 3 is a layout of the thin film transistor array of FIG. 2.

FIGS. 4A-4H is a set of steps suitable for constructing pixel electrodesand amorphous silicon thin-film transistors.

FIG. 5 illustrates pixel electrodes, color filters, and a black matrix.

FIG. 6 illustrates a schematic of the active matrix elements, pixelelectrode, photo TFT, readout TFT, and a black matrix.

FIG. 7 illustrates a pixel electrode, photo TFT, readout TFT, and ablack matrix.

FIG. 8 is a layout of the thin film transistor array of FIGS. 6 and 7.

FIG. 9 is a graph of the capacitive charge on the light sensitiveelements as a result of touching the display at high ambient lightingconditions.

FIG. 10 is a graph of the capacitive charge on the light sensitiveelements as a result of touching the display at low ambient lightingconditions.

FIG. 11 is a graph of the photo-currents in an amorphous silicon TFTarray.

FIG. 12 is a graph of the capacitive charge on the light sensitiveelements as a result of touching the display and providing light from alight wand.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

Referring to FIG. 1, a liquid crystal display (LCD) 50 (indicated by abracket) comprises generally, a backlight 52 and a light valve 54(indicated by a bracket). Since liquid crystals do not emit light, mostLCD panels are backlit with fluorescent tubes or arrays oflight-emitting diodes (LEDs) that are built into the sides or back ofthe panel. To disperse the light and obtain a more uniform intensityover the surface of the display, light from the backlight 52 typicallypasses through a diffuser 56 before impinging on the light valve 54.

The transmittance of light from the backlight 52 to the eye of a viewer58, observing an image displayed on the front of the panel, iscontrolled by the light valve 54. The light valve 54 normally includes apair of polarizers 60 and 62 separated by a layer of liquid crystals 64contained in a cell gap between the polarizers. Light from the backlight52 impinging on the first polarizer 62 comprises electromagnetic wavesvibrating in a plurality of planes. Only that portion of the lightvibrating in the plane of the optical axis of a polarizer passes throughthe polarizer. In an LCD light valve, the optical axes of the first 62and second 60 polarizer are typically arranged at an angle so that lightpassing through the first polarizer would normally be blocked frompassing through the second polarizer in the series. However, theorientation of the translucent crystals in the layer of liquid crystals64 can be locally controlled to either “twist” the vibratory plane ofthe light into alignment with the optical axes of the polarizer,permitting light to pass through the light valve creating a brightpicture element or pixel, or out of alignment with the optical axis ofone of the polarizes, attenuating the light and creating a darker areaof the screen or pixel.

The surfaces of the a first glass substrate 61 and a second glasssubstrate 63 form the walls of the cell gap are buffed to producemicroscopic grooves to physically align the molecules of liquid crystal64 immediately adjacent to the walls. Molecular forces cause adjacentliquid crystal molecules to attempt to align with their neighbors withthe result that the orientation of the molecules in the column ofmolecules spanning the cell gap twist over the length of the column.Likewise, the plane of vibration of light transiting the column ofmolecules will be “twisted” from the optical axis of the first polarizer62 to a plane determined by the orientation of the liquid crystals atthe opposite wall of the cell gap. If the wall of the cell gap is buffedto align adjacent crystals with the optical axis of the secondpolarizer, light from the backlight 52 can pass through the series ofpolarizers 60 and 62 to produce a lighted area of the display whenviewed from the front of the panel (a “normally white” LCD).

To darken a pixel and create an image, a voltage, typically controlledby a thin film transistor, is applied to an electrode in an array oftransparent electrodes deposited on the walls of the cell gap. Theliquid crystal molecules adjacent to the electrode are attracted by thefield produced by the voltage and rotate to align with the field. As themolecules of liquid crystal are rotated by the electric field, thecolumn of crystals is “untwisted,” and the optical axes of the crystalsadjacent to the cell wall are rotated progressively out of alignmentwith the optical axis of the corresponding polarizer progressivelyreducing the local transmittance of the light valve 54 and attenuatingthe luminance of the corresponding pixel. Conversely, the polarizers andbuffing of the light valve can be arranged to produce a “normally black”LCD having pixels that are dark (light is blocked) when the electrodesare not energized and light when the electrodes are energized. Color LCDdisplays are created by varying the intensity of transmitted light foreach of a plurality of primary color (typically, red, green, and blue)sub-pixels that make up a displayed pixel.

The aforementioned example was described with respect to a twistednematic device. However, this description is only an example and otherdevices may likewise be used, including but not limited to, multi-domainvertical alignment, patterned vertical alignment, in-plane switching,and super-twisted nematic type LCDs. In addition other devices, such asfor example, plasma displays, electroluminescent displays, liquidcrystal on silicon displays, reflective liquid crystal devices maylikewise be used. For such displays the light emitting portion of thedisplay, or portion of the display that permits the display of selectedportions of light may be considered to selectively cause the pixels toprovide light.

For an active matrix LCD (AMLCD) the inner surface of the second glasssubstrate 63 is normally coated with a continuous electrode while thefirst glass substrate 61 is patterned into individual pixel electrodes.The continuous electrode may be constructed using a transparentelectrode, such as indium tin oxide. The first glass substrate 61 mayinclude thin film transistors (TFTs) which act as individual switchesfor each pixel electrode (or group of pixel electrodes) corresponding toa pixel (or group of pixels). The TFTs are addressed by a set ofmultiplexed electrodes running along the gaps between the pixelelectrodes. Alternatively the pixel electrodes may be on a differentlayer from the TFTs. A pixel is addressed by applying voltage (orcurrent) to a select line which switches the TFT on and allows chargefrom the data line to flow onto the rear pixel electrodes. Thecombination of voltages between the front electrode and the pixelelectrodes sets up a voltage across the pixels and turns the respectivepixels on. The thin-film transistors are typically constructed fromamorphous silicon, while other types of switching devices may likewisebe used, such as for example, metal-insulator-metal diode andpolysilicon thin-film transistors. The TFT array and pixel electrodesmay alternatively be on the top of the liquid crystal material. Also thelight sensitive elements may likewise be located on the top of theliquid crystal material, if desired.

Referring to FIG. 2, the active matrix layer may include a set of datalines and a set of select lines. Normally one data line is included foreach column of pixels across the display and one select line is includedfor each row of pixels down the display, thereby creating an array ofconductive lines. To load the data to the respective pixels indicatingwhich pixels should be illuminated, normally in a row-by-row manner, aset of voltages are imposed on the respective data lines 204 whichimposes a voltage on the sources 202 of latching transistors 200. Theselection of a respective select line 210, interconnected to the gates212 of the respective latching transistors 200, permits the voltageimposed on the sources 202 to be passed to the drain 214 of the latchingtransistors 200. The drains 214 of the latching transistors 200 areelectrically connected to respective pixel electrodes and arecapacitively coupled to a respective common line 221 through arespective Cst capacitor 218. In addition, a respective capacitanceexists between the pixel electrodes enclosing the liquid crystalmaterial, noted as capacitances Clc 222 (between the pixel electrodesand the common electrode on the color plate). The common line 221provides a voltage reference. In other words, the voltage data(representative of the image to be displayed) is loaded into the datalines for a row of latching transistors 200 and imposing a voltage onthe select line 210 latches that data into the holding capacitors andhence the pixel electrodes.

Referring to FIG. 3, a schematic layout is shown of the active matrixlayer. The pixel electrodes 230 are generally grouped into a “single”effective pixel so that a corresponding set of pixel electrodes 230 maybe associated with respective color filters (e.g., red, green, blue).The latching transistors 200 interconnect the respective pixelelectrodes 230 with the data lines and the select line. The pixelelectrodes 230 may be interconnected to the common line 221 by thecapacitors Cst 218.

Referring to FIG. 4, the active matrix layer may be constructed using anamorphous silicon thin-film transistor fabrication process. The stepsmay include gate metal deposition (FIG. 4A), a photolithography/etch(FIG. 4B), a gate insulator and amorphous silicon deposition (FIG. 4C),a photolithography/etch (FIG. 4D), a source/drain metal deposition (FIG.4E), a photolithography/etch (FIG. 4F), an ITO deposition (FIG. 4G), anda photolithography/etch (FIG. 4H). Other processes may likewise be used,as desired.

The present inventors considered different potential architectural touchpanel schemes to incorporate additional optical layers between thepolarizer on the front of the liquid crystal display and the front ofthe display. These additional layers include, for example, glass plates,wire grids, transparent electrodes, plastic plates, spacers, and othermaterials. In addition, the present inventors considered the additionallayers with different optical characteristics, such as for example,birefringence, non-birefringence, narrow range of wavelengths, widerange of wavelengths, etc. After an extensive analysis of differentpotential configurations of the touch screen portion added to thedisplay together with materials having different optical properties andfurther being applied to the different types of technologies (e.g.,mechanical switches, crossed beams of infrared light, acoustic surfacewaves, capacitance sensing, and resistive membranes), the presentinventors determined that an optimized touch screen is merely a tradeoffbetween different undesirable properties. Accordingly, the design of anoptimized touch screen is an ultimately unsolvable task. In contrast todesigning an improved touch screen, the present inventors came to therealization that modification of the structure of the active matrixliquid crystal device itself could provide an improved touch screencapability without all of the attendant drawbacks to the touch screenconfiguration located on the front of the display.

Referring to FIG. 5, with particular attention to the latchingtransistors of the pixel electrodes, a black matrix 240 is overlying thelatching transistors so that significant ambient light does not strikethe transistors. Color filters 242 may be located above the pixelelectrodes. Ambient light striking the latching transistors results indraining the charge imposed on the pixel electrodes through thetransistor. The discharge of the charge imposed on the pixel electrodesresults in a decrease in the operational characteristics of the display,frequently to the extent that the display is rendered effectivelyinoperative. With the realization that amorphous silicon transistors aresensitive to light incident thereon, the present inventors determinedthat such transistors within the active matrix layer may be used as abasis upon which to detect the existence of or non-existence of ambientlight incident thereon (e.g., relative values thereto).

Referring to FIG. 6, a modified active matrix layer may include aphoto-sensitive structure or elements. The preferred photo-sensitivestructure includes a photo-sensitive thin film transistor (photo TFT)interconnected to a readout thin film transistor (readout TFT). Acapacitor Cst2 may interconnect the common line to the transistors.Referring to FIG. 7, a black matrix may be in an overlying relationshipto the readout TFT. The black matrix is preferably an opaque material orotherwise the structure of the display selectively inhibiting thetransmission of light to selective portions of the active matrix layer.Preferably the black matrix is completely overlying the amorphoussilicon portion of the readout TFT, and at least partially overlying theamorphous silicon portion of the readout TFT. Preferably the blackmatrix is completely non-overlying the amorphous silicon portion of thephoto TFT, and at least partially non-overlying the amorphous siliconportion of the photo TFT. Overlying does not necessarily denote directcontact between the layers, but is intended to denote in the generalsense the stacked structure of materials. In some embodiments, the blackmatrix inhibits ambient light from impacting the amorphous siliconportion of the readout TFT to an extent greater than inhibiting ambientlight from impacting the amorphous silicon portion of the photo TFT.

As an example, the common line may be set at a negative voltagepotential, such as −10 volts. During the previous readout cycle, avoltage is imposed on the select line which causes the voltage on thereadout line to be coupled to the drain of the photo TFT and the drainof the readout TFT, which results in a voltage potential across Cst2.The voltage coupled to the drain of the photo TFT and the drain of thereadout TFT is approximately ground (e.g., zero volts) with thenon-inverting input of the operational amplifier connected to ground.The voltage imposed on the select line is removed so that the readoutTFT will turn “off”.

Under normal operational conditions ambient light from the front of thedisplay passes through the black matrix and strikes the amorphoussilicon of the photo TFT. However, if a person touches the front of thedisplay in a region over the opening in the black matrix or otherwiseinhibits the passage of light through the front of the display in aregion over the opening in the black matrix, then the photo TFTtransistor will be in an “off” state. If the photo TFT is “off” then thevoltage across the capacitor Cst2 will not significantly dischargethrough the photo TFT. Accordingly, the charge imposed across Cst2 willbe substantially unchanged. In essence, the voltage imposed across Cst2will remain substantially unchanged if the ambient light is inhibitedfrom striking the photo TFT.

To determine the voltage across the capacitor Cst2, a voltage is imposedon the select line which causes the gate of the readout TFT tointerconnect the imposed voltage on Cst2 to the readout line. If thevoltage imposed on the readout line as a result of activating thereadout TFT is substantially unchanged, then the output of theoperational amplifier will be substantially unchanged (e.g., zero). Inthis manner, the system is able to determine whether the light to thedevice has been inhibited, in which case the system will determine thatthe screen has been touched at the corresponding portion of the displaywith the photo TFT.

During the readout cycle, the voltage imposed on the select line causesthe voltage on the respective drain of the photo TFT and the drain ofthe readout TFT to be coupled to the respective readout line, whichresults in resetting the voltage potential across Cst2. The voltagecoupled to the drain of the photo TFT and the drain of the readout TFTis approximately ground (e.g., zero volts) with the non-inverting inputof the operational amplifier connected to ground. The voltage imposed onthe select line is removed so that the readout TFT will turn “off”. Inthis manner, the act of reading the voltage simultaneously acts to resetthe voltage potential for the next cycle.

Under normal operational conditions ambient light from the front of thedisplay passes through the black matrix and strikes the amorphoussilicon of the photo TFT. If a person does not touch the front of thedisplay in a region over the opening in the black matrix or otherwiseinhibits the passage of light through the front of the display in aregion over the opening in the black matrix, then the photo TFTtransistor will be in an “on” state. If the photo TFT is “on” then thevoltage across the capacitor Cst2 will significantly discharge throughthe photo TFT, which is coupled to the common line. In essence thevoltage imposed across Cst2 will decrease toward the common voltage.Accordingly, the charge imposed across Cst2 will be substantiallychanged in the presence of ambient light. Moreover, there is asubstantial difference in the voltage potential across the holdcapacitor when the light is not inhibited versus when the light isinhibited.

Similarly, to determine the voltage across the capacitor Cst2, a voltageis imposed on the select line which causes the gate of the readout TFTto interconnect the imposed voltage to the readout line. If the voltageimposed on the readout line as a result of activating the readout TFT issubstantially changed or otherwise results in an injection of current,then the output of the operational amplifier will be substantiallynon-zero. The output voltage of the operational amplifier isproportional or otherwise associated with the charge on the capacitorCst2. In this manner, the system is able to determine whether the lightto the device has been uninhibited, in which case the system willdetermine that the screen has not been touched at the correspondingportion of the display with the photo TFT.

Referring to FIG. 8, a layout of the active matrix layer may include thephoto TFT, the capacitor Cst2, the readout TFT in a region between thepixel electrodes. Light sensitive elements are preferably included atselected intervals within the active matrix layer. In this manner, thedevice may include touch panel sensitivity without the need foradditional touch panel layers attached to the front of the display. Inaddition, the additional photo TFT, readout TFT, and capacitor may befabricated together with the remainder of the active matrix layer,without the need for specialized processing. Moreover, the complexity ofthe fabrication process is only slightly increased so that the resultingmanufacturing yield will remain substantially unchanged. It is to beunderstood that other light sensitive elements may likewise be used. Inaddition, it is to be understood that other light sensitive electricalarchitectures may likewise be used.

Referring to FIG. 11, a graph of the photo-currents within amorphoussilicon TFTs is illustrated. Line 300 illustrates a dark ambientenvironment with the gate connected to the source of the photo TFT. Itwill be noted that the leakage currents are low and relatively stableover a range of voltages. Line 302 illustrates a dark ambientenvironment with a floating gate of the photo TFT. It will be noted thatthe leakage currents are generally low and relatively unstable over arange of voltages (significant slope). Line 304 illustrates a lowambient environment with the gate connected to the source of the photoTFT. It will be noted that the leakage currents are three orders ofmagnitude higher than the corresponding dark ambient conditions andrelatively stable over a range of voltages. Line 306 illustrates a lowambient environment with a floating gate of the photo TFT. It will benoted that the leakage currents are generally three orders of magnitudehigher and relatively unstable over a range of voltages (significantslope). Line 308 illustrates a high ambient environment with the gateconnected to the source of the photo TFT. It will be noted that theleakage currents are 4.5 orders of magnitude higher than thecorresponding dark ambient conditions and relatively stable over a rangeof voltages. Line 310 illustrates a high ambient environment with afloating gate of the photo TFT. It will be noted that the leakagecurrents are generally 4.5 orders of magnitude higher and relativelyunstable over a range of voltages (significant slope). With thesignificant difference between the dark state, the low ambient state,and the high ambient state, together with the substantially flatresponses over a voltage range (source-drain voltage), the system mayreadily process the data in a confident manner, especially with the gateconnected to the source.

Referring to FIG. 9, under high ambient lighting conditions the photoTFT will tend to completely discharge the Cst2 capacitor to the commonvoltage, perhaps with an offset voltage because of the photo TFT. Inthis manner, all of the photo TFTs across the display will tend todischarge to the same voltage level. Those regions with reduced ambientlighting conditions or otherwise where the user blocks ambient lightfrom reaching the display, the Cst2 capacitor will not fully discharge,as illustrated by the downward spike in the graph. The downward spike inthe graph provides location information related to the region of thedisplay that has been touched.

Referring to FIG. 10, under lower ambient lighting conditions the photoTFT will tend to partially discharge the Cst2 capacitor to the commonvoltage. In this manner, all of the photo TFTs across the display willtend to discharge to some intermediate voltage levels. Those regionswith further reduced ambient lighting conditions or otherwise where theuser blocks ambient light from reaching the display, the Cst2 capacitorwill discharge to a significantly less extent, as illustrated by thedownward spike in the graph. The downward spike in the graph provideslocation information related to the region of the display that has beentouched. As shown in FIGS. 9 and 10, the region or regions where theuser inhibits light from reaching the display may be determined aslocalized minimums. In other embodiments, depending on the circuittopology, the location(s) where the user inhibits light from reachingthe display may be determined as localized maximums or otherwise somemeasure from the additional components.

In the circuit topology illustrated, the value of the capacitor Cst2 maybe selected such that it is suitable for high ambient lightingconditions or low ambient lighting conditions. For low ambient lightingconditions, a smaller capacitance may be selected so that the device ismore sensitive to changes in light. For high ambient lightingconditions, a larger capacitance may be selected so that the device isless sensitive to changes in light. In addition, the dimensions of thephoto transistor may be selected to change the photo-leakage current.Also, one set of light sensitive elements (e.g., the photo TFT and thecapacitance) within the display may be optimized for low ambientlighting conditions while another set of light sensitive elements (e.g.,the photo TFT and the capacitance) within the display may be optimizedfor high ambient lighting conditions. Typically, the data from lightsensitive elements for low ambient conditions and the data from lightsensitive elements for high ambient conditions are separately processed,and the suitable set of data is selected. In this manner, the samedisplay device may be used for high and low ambient lighting conditions.In addition, multiple levels of sensitivity may be provided. It is to beunderstood that a single architecture may be provided with a wide rangeof sensitivity from low to high ambient lighting conditions.

Another structure that may be included is selecting the value of thecapacitance so that under normal ambient lighting conditions the chargeon the capacitor only partially discharges. With a structure where thecapacitive charge only partially discharges, the present inventorsdetermined that an optical pointing device, such as a light wand orlaser pointer, may be used to point at the display to further dischargeparticular regions of the display. In this manner, the region of thedisplay that the optical pointing device remains pointed at may bedetected as local maximums (or otherwise). In addition, those regions ofthe display where light is inhibited will appear as local minimums (orotherwise). This provides the capability of detecting not only theabsence of light (e.g., touching the panel) but likewise those regionsof the display that have increased light incident thereon. Referring toFIG. 12, a graph illustrates local minimums (upward peaks) from addedlight and local maximums (downward peaks) from a lack of light. Inaddition, one set of light sensitive elements (e.g., the photo TFT andthe capacitance) within the display may be optimized for ambientlighting conditions to detect the absence of light while another set oflight sensitive elements (e.g., the photo TFT and the capacitance)within the display may be optimized for ambient lighting conditions todetect the additional light imposed thereon.

A switch associated with the display may be provided to select among aplurality of different sets of light sensitive elements. For example,one of the switches may select between low, medium, and high ambientlighting conditions. For example, another switch may select between atouch sensitive operation (absence of light) and a optical pointingdevice (addition of light). In addition, the optical pointing device maycommunicate to the display, such as through a wire or wirelessconnection, to automatically change to the optical sensing mode.

It is noted that the teachings herein are likewise applicable totransmissive active matrix liquid crystal devices, reflective activematrix liquid crystal devices, transflective active matrix liquidcrystal devices, etc. In addition, the light sensitive elements maylikewise be provided within a passive liquid crystal display. Thesensing devices may be, for example, photo resistors and photo diodes.

Alternatively, light sensitive elements may be provided between the rearpolarizing element and active matrix layer. In this arrangement, thelight sensitive elements are preferably fabricated on the polarizer, orotherwise a film attached to the polarizer. In addition, the lightsensitive elements may be provided on a thin glass plate between thepolarizer and the liquid crystal material. In addition, the black matrixor otherwise light inhibiting material is preferably arranged so as toinhibit ambient light from striking the readout TFT while free frominhibiting light from striking the photo TFT. Moreover, preferably alight blocking material is provided between the photo TFT and/or thereadout TFT and the backlight, such as gate metal, if provided, toinhibit the light from the backlight from reaching the photo TFT and/orthe readout TFT.

Alternatively, light sensitive elements may be provided between thefront polarizing element and the liquid crystal material. In thisarrangement, the light sensitive elements are preferably fabricated onthe polarizer, or otherwise a film attached to the polarizer. Inaddition, the light sensitive elements may be provided on a thin glassplate between the polarizer and the liquid crystal material. The lightsensitive elements may likewise be fabricated within the front electrodelayer by patterning the front electrode layer and including suitablefabrication techniques. In addition, a black matrix or otherwise lightinhibiting material is preferably arranged so as to inhibit ambientlight from striking the readout TFT while free from inhibiting lightfrom striking the photo TFT. Moreover, preferably a light blockingmaterial is provided between the photo TFT and/or the readout TFT andthe backlight, if provided, to inhibit the light from the backlight fromreaching the photo TFT and/or the readout TFT.

Alternatively, light sensitive elements may be provided between thefront of the display and the rear of the display, normally fabricated onone of the layers therein or fabricated on a separate layer providedwithin the stack of layers within the display. In the case of a liquidcrystal device with a backlight the light sensitive elements arepreferably provided between the front of the display and the backlightmaterial. The position of the light sensitive elements are preferablybetween (or at least partially) the pixel electrodes, when viewed from aplan view of the display. This may be particularly useful for reflectivedisplays where the pixel electrodes are opaque. In this arrangement, thelight sensitive elements are preferably fabricated on one or more of thelayers, or otherwise a plate attached to one or more of the layers. Inaddition, a black matrix or otherwise light inhibiting material ispreferably arranged so as to inhibit ambient light from striking thereadout TFT while free from inhibiting light from striking the photoTFT. Moreover, preferably a light blocking material is provided betweenthe photo TFT and/or the readout TFT and the backlight, if provided, toinhibit the light from the backlight from reaching the photo TFT and/orthe readout TFT.

In many applications it is desirable to modify the intensity of thebacklight for different lighting conditions. For example, in darkambient lighting conditions it may be beneficial to have a dimbacklight. In contrast, in bright ambient lighting conditions it may bebeneficial to have a bright backlight. The integrated light sensitiveelements within the display stack may be used as a measure of theambient lighting conditions to control the intensity of the backlightwithout the need for an additional external photo-sensor. One lightsensitive element may be used, or a plurality of light sensitive elementmay be used together with additional processing, such as averaging.

The terms and expressions which have been employed in the foregoingspecification are used therein as terms of description and not oflimitation, and there is no intention, in the use of such terms andexpressions, of excluding equivalents of the features shown anddescribed or portions thereof, it being recognized that the scope of theinvention is defined and limited only by the claims which follow.

1. A liquid crystal device comprising: (a) a front electrode layer; (b)a rear electrode layer; (c) a liquid crystal material located betweensaid front electrode layer and said rear electrode layer; (d) changingan electrical potential between said rear electrode layer and said frontelectrode layer to selectively modify portions of said liquid crystalmaterial to change the polarization of the light incident thereon; (e) aplurality of light sensitive elements located together with said rearelectrode layer; and (f) wherein each of said light sensitive elementsinclude a transistor; (g) wherein each of said light sensitive elementsincludes a first transistor that senses ambient light, and a secondtransistor that is inhibited from sensing ambient light with respect tosaid first transistor; (h) wherein a terminal of said first transistoris connected to a terminal of said second transistor with a firstconductor; and (i) wherein said first conductor is capacitively coupledto a common line wherein said light sensitive elements determine achange in the light level to a level less than the ambient light level.2. The device of claim 1 wherein said common line has a voltagepotential less than said first conductor.
 3. A liquid crystal devicecomprising: (a) a front electrode layer; (b) a rear electrode layercomprising a plurality of pixel electrodes; (c) a liquid crystalmaterial located between said front electrode layer and said rearelectrode layer; (d) a plurality of light sensitive elements locatedwithin said display wherein each of said light sensitive elementsincludes a first transistor that senses ambient light, a secondtransistor that is inhibited from sensing ambient light with respect tosaid first transistor; (e) wherein a terminal of said first transistoris connected to a terminal of said second transistor with a firstconductor; and (f) wherein said first conductor is capacitively coupledto another line with a capacitor wherein said light sensitive elementsdetermine a change in the light level to a level less than the ambientlight level.